/Engine/Source/Editor/UnrealEd/Classes/MaterialGraph/MaterialGraph.h
Add a Subgraph to the Graph
UMaterialGraph* Newly created subgraph
[UMaterialGraph](API\Editor\UnrealEd\MaterialGraph\UMaterialGraph) * AddSubGraph ( [UMaterialExpression](API\Runtime\Engine\Materials\UMaterialExpression) * InSubgraphExpression )